1. Field of the Invention
The present invention relates to a developing apparatus for use in an image forming apparatus, such as a photocopier, a facsimile, a printer, etc., and more particularly to a developing apparatus and method of determining an end-of-toner condition in a toner storage unit.
2. Discussion of the Background
As a background image forming apparatus and a background developing apparatus which determine an end-of-toner condition in a toner storage unit, Japanese Laid-open Patent Publication No. 9-120209 describes an end-of-toner detection device that includes (1) a light path forming unit that protrudes from a wall of a toner storage unit to form a light path along which a light beam passes through an inside of the toner storage unit, and (2) a light sensor of a light-transmission type that includes a light-emitting unit that emits light to a first light-reflecting surface of the light path forming unit and a light-receiving unit that receives the light reflected from a second light-reflecting surface of the light path forming unit. In the above-described end-of-toner detection device, when toner remains in the toner storage unit, the light emitted from the light-emitting unit is blocked by the toner in the light path and does not reach the light-receiving unit. As the amount of toner decreases in the toner storage unit, the intensity of light received by the light-receiving unit increases. In the light sensor, a reference value of the intensity of light received by the light-receiving unit is fixed to detect the end-of-toner condition in the toner storage unit. The light sensor compares the intensity of light received by the light-receiving unit with the reference value, and sends binary output signals (Level High/Level Low) to indicate the end-of-toner condition in the toner storage unit.
However, when the reference value for detecting the end-of-toner condition is fixed in the light sensor, an end-of-toner detection error may be caused by factors such as unevenness of sensitivity of the light sensor, color of toner, etc. For example, when the light sensor has higher sensitivity, the light-receiving unit of the light sensor may receive more light and may determine the end-of-toner condition even though toner remains in the toner storage unit. Further, when the light sensor is stained by scattered toner, the intensity of light received by the light-receiving unit may change, so that an end-of-toner determination error may be caused. Furthermore, because each color toner has a different characteristic of light transmission factor, the intensity of light received by the light-receiving unit may change according to the color of toner, so that an end-of-toner determination error may be caused by the end-of-toner detection employing a fixed reference value.